We plan to exploit the unique features of the hamster photoperiodic response in two general ways. (1) Treating the animal as a black box, we plan to analyze photoperiodic time measurement in detail. The primary objective of this portion of our work will be the elucidation of the hamster circadian photoperiodic response curve. This curve will enable us to predict the effect on the reproductive system of short pulses of light presented at any specific time in the animal's day. The methods we will use involve a combination of techniques from the field of circadian analysis with modern endocrinological methods especially the radioimmunological assays for gonadotropins which provide exceptionally useful end points. (2) At the same time we will take a physiological approach to hamster photoperiodism, working toward a description and analysis of the endocrine events which lead, some 3 weeks after the onset of photoperiodic stimulation, to an effect on the gonads. We will search for rhythmic response to light in hypothalamic releasing factor mechanisms, and pituitary and plasma levels of both LH and FSH. Great analytic power is available for these studies as a result of our ability to produce major effects on the reproductive system with short duration light pulses applied at specific times in the circadian cycle. By integrating information obtained from these two different approaches, we hope to construct a detailed picture of the action of light in controlling the reproductive system of a mammalian species which has great potential as a laboratory model.